Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as an energy source. Among these secondary batteries, lithium secondary batteries having high energy density and voltage are commercially available now. The lithium secondary batteries generally use a lithium transition metal oxide as a cathode active material and a graphite-based material as an anode active material, whereby charge and discharge is performed via the repeated intercalation/deintercalation process of lithium ions from the cathode to the anode or vice versa.
In recent years, a number of studies and suggestions have been focused on anode active materials of metallic materials such as silicon and tin, as they are known to be capable of performing reversible intercalation and deintercalation of large amounts of lithium ions through the reaction with lithium.
Generally, even though there is a difference in the theoretical capacity of the battery depending upon kinds of electrode active materials, the charge/discharge capacity of the battery usually decreases as the charge/discharge cycle is repeated. The primary cause of such a capacity decrease is a failure to sufficiently fulfill functions of the electrode active material due to separation between the electrode active materials and/or between the electrode active material and current collector, resulting from volume changes of electrodes occurring during repeated charge/discharge cycles of the battery. Further, since the lithium ions intercalated into the anode are not sufficiently and smoothly released from the anode during the intercalation and deintercalation process, the active points of the anode are decreased as charge/discharge cycles are repeated. Consequently, a further progress of charge/discharge cycles also leads to decreases in the charge/discharge capacity and life characteristics of the battery.
In particular, when natural graphite having a theoretical discharge capacity of 372 mAh/g is used in admixture with a high-discharge capacity material such as silicon, tin, silicon/tin alloy or silicon/carbon composite, in order to improve the discharge capacity, the repeated charge and discharge cycles lead to significant increases in volume expansion of the electrode materials, which consequently results in separation of the electrode mix from the current collector and thereby a sharp drop of the battery capacity even after several or several tens of cycles.
As such, there is an urgent need in the art for the development of a binder and electrode mix which are capable of achieving a structural stability of the electrode by controlling volume expansion of the electrode active materials which may occur during the repeated charge and discharge cycles, and are thus capable of improving the battery performance.
At present, polyvinylidene fluoride (PVdF), which is currently widely used as a binder of cathode and anode, is a polymer resin soluble in an organic solvent such as N-methyl pyrrolidone (NMP). PVdF was not used as an adhesive at first. However, PVdF is now widely used as a binder of electrode active materials since it exhibits good miscibility with a graphite material and it is possible to manufacture an electrode plate having high adhesive strength by addition of PVdF in an amount of about 8 to 10% of graphite.
However, PVdF covers the electrode active material in the state densely packed with a polymer resin and therefore deteriorates the native performance of the electrode active material in terms of the capacity and efficiency of the battery. In addition, due to poor softness, PVdF is susceptible to cleavage of bonds and deterioration of cycle characteristics, when a material having a large specific surface area and a high expansion/shrinkage rate upon charge/discharge cycling, such as natural graphite or a metallic active material, is used as the electrode active material. Further, PVdF tends to undergo expansion by absorption of a carbonate-based electrolyte, and therefore exhibits a significant decrease of the output capacity as charge/discharge cycles continue to be repeated.
As another type of a binder which is used in lithium secondary batteries, there is rubber-based latex such as styrene butadiene rubber (SBR), as aqueous binder. SBR has high elasticity and is recognized to improve the capacity and initial charge/discharge efficiency of the secondary battery using SBR. However, SBR has relatively low adhesive strength and therefore suffers from limited applications in that SBR cannot be employed in high-capacity active materials such as metallic active materials, undergoing significant volume expansion upon charge/discharge cycling and thus requiring an electrode having high adhesive strength.
Meanwhile, as a further example of a binder for an electrode mix, the use of a polyvinyl alcohol solution has been attempted. However, as can be confirmed from comparative experiments of Japanese Patent Laid-open Publication No. 2003-109596, it is known that the single use of the polyvinyl alcohol is not satisfactory as the binder for the electrode mix due to a low viscosity, non-uniform application of the binder onto metal foil as a current collector, and relatively low adhesive strength. Further, it was confirmed that the use of the polyvinyl alcohol binder suffers from a decreased output power due to a large voltage drop upon performing high-rate charge/discharge and a process disadvantage associated with the heat treatment necessary to improve adhesion between the electrode mix and current collector (Japanese Patent Laid-open Publication No. 2004-134208). Further, it was also confirmed that the use of a polyvinyl alcohol resin disadvantageously exhibits a short high-temperature life of a secondary battery due to a poor electrolyte resistance at a high temperature of 50° C. which is the upper limit of a serviceable temperature (Japanese Patent Laid-open Publication No. 2003-157851).
Despite many teachings that propose the usability of polyvinyl alcohol as the binder, the fundamental reason of difficulty to use the polyvinyl alcohol alone have not yet been fully elucidated. The various attempts and efforts have been made to overcome the limitations as discussed above.
For instance, there have been proposed a variety of technical arts for improving physical properties of the polyvinyl alcohol, by the use of the polyvinyl alcohol in admixture with other polymer resins (Japanese Patent Laid-open Publication Nos. Hei 11-67215, 2003-109596 and 2004-134208), copolymerization of the polyvinyl alcohol with other monomers (Japanese Patent Laid-open Publication No. 1999-250915), modification of the terminal groups of the polyvinyl alcohol (Japanese Patent Laid-open Publication No. 2004-134369), or the like.
In this connection, Japanese Patent Laid-open Publication No. Hei 11-67215 has proposed the use of a water-soluble polymer as a binder for an anode, for example, a polyvinyl alcohol having a degree of polymerization (DP) of 1700. However, the inventors of the present invention have confirmed that the prolonged use of the polyvinyl alcohol having a polymerization degree of 1700 results in severe deterioration of the battery performance due to low electrolyte resistance, and particularly worsening of binder dissolution in the electrolyte upon continuous charge/discharge cycling at a high temperature. The secondary batteries easily reach a high temperature (for example, around 50° C.) during the continuous discharge process and the ensuing significant deterioration of the high-temperature performance may be an obstacle to impede the use of the secondary battery per se. Therefore, despite various suggestions of the conventional prior arts as discussed above, the long-term use of the polyvinyl alcohol as the binder has suffered from severe degradation of the battery performance, the fact of which can also be confirmed in the following examples.